1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission.
2. Discussion of the Background
A hydraulic control apparatus for an automatic transmission of related art that sets one of a plurality of speed stages with different gear ratios by selectively engaging and disengaging a plurality of hydraulic frictional engagement elements includes a plurality of linear solenoid valves to regulate fluid pressures of automatic transmission fluid that is supplied to the plurality of frictional engagement elements.
The hydraulic control apparatus for the automatic transmission uses one or more linear solenoid valves to set one of the speed stages (for example, a first speed (LOW gear)). However, the linear solenoid valves typically include a linear solenoid valve that is not used. For example, when a linear solenoid valve of a normally closed type is used, by setting an exciting current to zero, the consumption flow of the automatic transmission fluid by the unused linear solenoid valve can be decreased, and energy efficiency can be increased.
However, if the exciting current is set to zero, since a spool (a valve element) of the linear solenoid valve is held at a movement end at which the output of the fluid pressure is stopped, it takes a time until the spool is actually moved and balanced. The responsiveness of the fluid pressure may be degraded, and hence the responsiveness of the speed change may be degraded.
To address this, there is provided a hydraulic control apparatus for an automatic transmission, for example, disclosed in Japanese Unexamined Patent Application Publication No. 2005-321014 (hereinafter, referred to as the document '014). The hydraulic control apparatus uses the balance of a spool to change the state between a regulated state in which a fluid pressure corresponding to an electromagnetic force of a solenoid, and a not-regulated state in which the spool is held at a movement end at which the output of the fluid pressure is stopped. When the speed change of the automatic transmission is predicted or determined, the state of an unused linear solenoid valve, which regulates the fluid pressure of a hydraulic frictional engagement element to be newly engaged for the speed change, is changed to the regulated state before the speed change.
The hydraulic control apparatus for the automatic transmission disclosed in the document '014, the unused linear solenoid valve is in a minimally regulated state if an AT fluid temperature is a predetermined value or lower, and the unused linear solenoid valve is in a not-regulated state if the AT fluid temperature is higher than the predetermined value. With the control, the responsiveness of the fluid pressure is increased when the viscosity of the automatic transmission fluid is high, and the leaking flow is decreased when the viscosity of the automatic transmission fluid is low. Thus, the load on an engine can be decreased, and the fuel economy (fuel consumption efficiency) of a vehicle can be improved.
The hydraulic control apparatus for the automatic transmission disclosed in the document '014 controls the unused linear solenoid valve to be in the not-regulated state when the viscosity of the automatic transmission fluid is low or when the temperature of the automatic transmission fluid is high, however, the responsiveness may be degraded when the linear solenoid valve is used from the not-regulated state.
The hydraulic control apparatus for the automatic transmission disclosed in the document '014 controls the unused linear solenoid valve to be in the minimally regulated state when the viscosity of the automatic transmission fluid is high or when the temperature of the automatic transmission fluid is low, however, since the linear solenoid valve is in the minimally regulated state, a wall surface of an input port connected with a hydraulic pump contacts (or is located near) a wall surface of a spool groove. The automatic transmission fluid may flow out (leak). The hydraulic pump typically has a pump characteristic such that the discharge flow of the hydraulic pump changes depending on the engine speed. If the engine speed is low, the discharge flow of the hydraulic pump is low. Thus, if the consumption flow at the unused linear solenoid valve is high, the quantity of the automatic transmission fluid or lubricant may be smaller than the required quantity.